This is the logo image in the BMW kit below:


If you're satisfied with the image's appearance then on the understanding that i've taken every care and precaution in the preparation of the SD flash kits however any flashing involves some risk and you assume all of that, then proceed as follows:
Note - Flash kit No.1 will backup the original logo region (16) and the non-volatile memory region (41) so keep the resultant 16_backup.bin and 41_backup.bin in safe permanent storage.

• Extract [Please Register or Login to download file] to a CLEAN microSD card formatted FAT16 or FAT32 only
  Ensure the nuvi is completely OFF, not just in standby
  Insert the card and power it on, if battery is low or weak connect it to a suitable 5V power supply which will power it on but don't use a computer USB port
  When the flash finishes the nuvi will reboot itself so remove the card. Check that 16_backup.bin and 41_backup.bin files have been created in the card root.

You may now flash the BMW logo using Flash Kit No.2:

• Extract [Please Register or Login to download file] to a CLEAN microSD (if you use the the same card be sure it's completely empty beforehand - reformat it if unsure)
  Insert the card and power it on, if battery is low or weak connect it to a suitable 5V power supply which will power it on but don't use a computer USB port
  When the flash finishes the nuvi will reboot itself and you can remove the card. It should now be displaying the BMW splash screen.

All done, no problems at all. Thank you very much for your help.

One quick question though - if ever I need to revert back to the Garmin logo then all I need to do is rename the 16_backup.bin to 16.bin and re-insert the sd card?

Can I use this on other 37xx units (as long as I take a backup beforehand)?

You can indeed revert back to original "Garmin" logo by using the backup file - i.e. "16_backup.bin" renamed and replacing the BMW "16.bin", just as you've correctly surmised. Do the same procedure otherwise: Make fresh backups of regions 16 and 41 (or at very least 41) using flash kit #1 first before using flash kit #2 with the replacement 16.bin. Note also that there's a file "last_id.bin" created during a flash in the 1101 folder, that BIN cannot be present for any further use of the card and must be removed if you want to do multiple flashes with the same card. The other created file "update.log" can be safely left or removed too.

It can be used on other 3700 series devices but only if they are also HWID 1101 US/EU firmware (Western markets) units. Some 3700 devices sold in other markets may have hardware differences. For instance Asian TN, SG, TH, ID are some examples of 3700's with different HWIDs. Using the supplied kits unmodified on those device might brick them, perhaps irretrievably due to incompatible ramloaders (Ldr.bin/boot.bin). The Ldr.bin would need to be replaced suitably and the HWID folder renamed - but then even so, there may be data differences in region 16 incompatible with the BMW logo i supplied which is meant for a Western device. It would be unwise to try modding the flash kit for any device other than 1101 unless you know exactly what you're doing.